Full email mailuser login not working

Hello guys,
I have a strange issue with my ISPConfig mailuser login (https://mydomain:81/mailuser/).
If I try to login with a ispconfig login, like web9_user , I can login successfully ;
If I try to login with the same user full email address, like user@domain , I get an "unknown user" error.
Any idea?

But if i want connect to the mailuser with the email address i have "invalid user"

Click to expand...

Then you will have to reconfigure your mail system to allow email addresses as username for pop3 and smtp in general by either switching to dovecot as pop3 mail server or by installing several patches to allow usernames with @ inside for courier.

Additionally, the configuration changes will most likely not affect any users craeted before you installed the patches.

Then you will have to reconfigure your mail system to allow email addresses as username for pop3 and smtp in general by either switching to dovecot as pop3 mail server or by installing several patches to allow usernames with @ inside for courier.

Additionally, the configuration changes will most likely not affect any users craeted before you installed the patches.

Click to expand...

Well, is there any step-by-step how-to, or maybe additional documentation on this process?
I think it would be a very useful feature to let the mail users login with their mail address.